The present invention relates to a sheet conveying smoothly apparatus to convey sheets discharged from a curl correcting device (decurling mechanism) to correct curls formed on the sheets.
In recent years, a wide width nip type fixing device has been widely used. In the fixing device, unfixed sheets are made to pass through a wide width nip formed between a hard roller and a pressing soft roller or between a hard roller and a pressing endless belt suspended around a plurality of rollers. Accordingly, this type fixing device has a tendency to form a large curl on a sheet after fixing. Then, a curl preventing technique has been proposed to employ a decurling mechanism to form an opposite curl to eliminate a deformation (curl) on a sheet caused by a fixing nip at the downstream side of a fixing section or in a sheet conveying apparatus located at the downstream side of a fixing device.
Japanese Patent Unexamined Publication No. 2003-295657 discloses a technique regarding a fixing apparatus in which at the downstream side of a fixing section to form a concave shaped fixing nip between a fixing roller and a fixing belt, a decurling section is arranged so as to form a convex shaped decurling nip between a conveying roller made of a hard material and a pressing roller made of a soft material in the direction reverse to the concave shaped fixing nip.
Japanese Patent Unexamined Publication No. 2006-168940 discloses the following curl correcting technique. As shown in FIG. 1 of this publication, a curl correcting mechanism is arranged between a reversing gate section and a reversing mechanism 56 on a both side print conveyance passage 57 at the downstream side of a fixing device 40. FIG. 2 of the publication shows an outlined structural view of the curl correcting mechanism. In this view, a sheet conveying nip formed between a rigid roller 71 and an elastic roller 72 is made to move around the rigid roller 71 so as to change an entering angle (discharging angle) at which a fixed sheet enters onto the surface of an inclined guide section 77a located at the downstream side of the sheet conveying nip, whereby a curl correcting function is controlled for the fixed sheet. Therefore, with the control to switch the entering angle in accordance with the kind of sheets, this curl correcting technique tries to make possible to correct a curl on a fixed sheet properly for various broad kinds of sheets from a thin sheet to a thick sheet without creating a curl in the reverse direction.
When the above-mentioned decurling mechanism was arranged at the downstream side of a fixing nip or a fixing device, a curl correcting function might be made surely. However, in order to realize a compact sheet conveying device, if a guide plate to regulate sheets and conveying rollers to convey sheets were arranged at the downstream side of the decurling mechanism, the following trouble occurred. That is, when the leading edge of a sheet was passing through the conveying rollers, irregular deformation, waving deformation, curl or the like took place on an area of the sheet locating at the upstream side of the decurling mechanism. The situation of the trouble seemed to tend to become more serious toward the trailing edge of the sheet. Further, although a both side printing job had been conducted normally for a while immediately after the job had started, trouble has been caused with the progress of the printing process, and the situation of the trouble has seemed to tend to become more serious gradually. Further, as soon as an elastic roller of the decurling mechanism was replaced with another one, abnormality occurred. Reversely, the situation that the device unexpectedly returned to normal condition was also observed.
Then, as a result of having repeated various tests and examinations, it has turned out that the main cause of the trouble was a mismatch between the sheet conveying speed of the decurling mechanism and that of the conveying rollers. Here, a method might be considered to match the both speeds constantly under various conditions by controlling the driving of the decurling mechanism independently of that of the conveying roller. However, the method caused another problem in the aspects of the compactness of the device and the cost. Therefore, there has been a desire for a sheet conveying apparatus excellent in compactness and capable of preventing trouble such as waving deformation, wrinkles and the like on a sheet.